1. Field of the Invention:
This invention relates to a silver catalyst to be used for the production of ethylene oxide by the catalytic gas-phase oxidation of ethylene with molecular oxygen and to a method for the manufacture of the silver catalyst.
2. Description of the Prior Art:
The silver catalyst which is used in the commercial production of ethylene oxide by the catalytic gas-phase oxidation of ethylene with molecular oxygen is required, for satisfactory performance of the function thereof, to exhibit high selectivity and high activity and enjoy a long catalyst life as.
Various studies have been made to date for the purpose of improving the performance of the silver catalyst and consequently fulfilling the requirement and efforts have been made to improve carriers, reaction promoters, silver compounds and the like. Numerous reports covering carriers have been published. The specifications of U.S. Pat. Nos. 3,207,700, 4,368,144, 2,766,261, 3,172,893 and 3,664,970, and the specifications of Japanese patent publication SHO No. 43(1968)-13,137, SHO No. 45(1970)-22,419 and SHO No. 45(1970)-11,217 are their examples. Most of them, however, concern pore distributions and specific surface areas of carriers.
In the specification of U.S. Pat. No. 2,125,333, there is a description to the effect that an alkali metal salt containing sodium or potassium and a metal salt thereof is used as an additive for the silver catalyst in the manufacture of ethylene oxide.
In the specification of U.S. Pat. No. 2,238,474, there is a description to the effect that sodium hydroxide improves the activity of the silver catalyst for the production of ethylene oxide and potassium hydroxide has an adverse effect upon the activity of the silver catalyst.
In the specification of U.S. Pat. No. 2,765,283, there is a description to the effect that the silver catalyst is improved by adding 1 to 2,000 ppm by weight of an inorganic chlorinated substance such as sodium chloride to the catalyst carrier before silver is deposited on the carrier.
In the specification of U.S. Pat. No. 2,799,687, there is a description to the effect that a halide such as sodium chloride or potassium chloride, used in an amount of 20 to 16,000 ppm, functions as an inhibitor and induces degradation of the activity of the silver catalyst.
In the specification of U.S. Pat. No. 4,007,135, there is disclosed a catalyst for the production of alkylene oxide, which catalyst contains copper, gold, zinc, cadmium, mercury, niobium, tantalum, molybdenum, tungsten, vanadium, or desirably chromium, calcium, magnesium, strontium, and/or more desirably barium, and preferably further an alkali metal, in an amount preceding the amount naturally present as an impurity or cement in the carrier and sufficient to manifest the action of a promoter.
In the specification of U.S. Pat. No. 4,168,247, there is disclosed a catalyst for the production of alkylene oxide, which catalyst contains silver deposited on a porous heat-resistant carrier possessing a specific surface area in the range of 0.05 to 10 m.sup.2 /g and further contains sodium and at least one other alkali metal selected from the group consisting of potassium, rubidium, and cesium in a promoting amount in excess of the amount naturally present as an impurity or a binding agent in the carrier.
In the specification of U.S. Pat. No. 4,278,562, there is disclosure to the effect that a catalyst for the production of an alkylene oxide is obtained by depositing silver and optionally sodium or lithium in the form of corresponding salts on a carrier, heating the carrier and in the subsequent treatment, depositing thereon the salts of such alkali metals as potassium, rubidium, and cesium in conjunction with an amine and/or ammonia.
In Japanese Patent Laid-Open No. SHO 55(1980)-145,677, there is disclosed a silver catalyst which, as a catalyst for the reaction of oxidation, has silver and, when necessary, further an alkali metal component or an alkaline earth metal component deposited on a non-acidic carrier containing alumina, silica, and titania in a total amunt of not less than 99% by weight, containing metal of the Groups Va, VIa, VIIa, VIII, Ib, and IIb of the Periodic Table of Elements in a total amount of less than 0.1% by weight, and assuming no acid color on exposure to methyl red having a pKa value of +4.8.
In Japanese Patent Laid-Open No. SHO 56(1981)-105,750, there is disclosed a silver catalyst for the production of ethylene oxide, which silver catalyst is prepared by impregnating a carrier using .alpha.-alumina as a principal component thereof and having a sodium content of not more than 0.07% by weight and a specific surface area in the range of 1 to 5 m.sup.2 /g with an impregnation having 0.001 to 0.05 gram equivalent, per kg of complete catalyst, of a complex of an alkali metal with boron, a complex of an alkali metal with molybdenum, and/or a complex of an alkali metal with tungsten contained in a decomposable silver solution formulated to give a deposition ratio of 5 to 25% by weight based on the complete catalyst, and then heating and reducing or thermally decomposing the product of impregnation.
In Japanese Patent Laid-Open No. SHO 57(1982)-107,241, there is disclosed a silver catalyst for the production of ethylene oxide, which catalyst incorporates therein, besides silver, sodium (Na) as a cationic component and chlorine (C1) as an anionic component in amounts such that the atomic ratio of C1/Na will be less than 1.
In the specification of U.S. Pat. No. 4,415,476, there is disclosed a silver catalyst for the production of ethylene oxide, which silver catalyst contains, besides silver, at least sodium and cesium as cationic components and chlorine as an anionic component.
In Japanese Patent Laid-Open No. SHO 57(1982)-171,435, there is disclosed a silver catalyst for the production of ethylene oxide, which silver catalyst contains metallic silver particles deposited in a ratio of 5 to 25% by weight based on complete catalyst on an .alpha.-alumina carrier having a sodium content of not more than 0.07% by weight and a specific surface area in the range of 0.5 to 5 m.sup.2 /g and 0.001 to 0.05 gram equivalent of at least one alkali metal or alkali metal compound per kg of the complete catalyst and in excess of the amount naturally present in the carrier.
In the specification of U.S. Pat. No. 4,248,740, there is disclosed a method for the manufacture of a silver catalyst for the production of ethylene oxide, which method is characterized by impregnating a porous inorganic refractory carrier with a silver compound containing a reducing substance, thermally reducing the resulting product of impregnation thereby causing fine silver particles to be dispersed and deposited on the outer surface of the carrier and on the inner walls of the pores in the carrier, subsequently washing the composite with water and/or a lower alcohol, drying the wet composite, further impregnating the composite with a solution of a reaction promoting substance, and evaporation the impregnated composite to dryness.
In the specification of EP No. EP-85237, there is disclosed a catalyst for the production of an alkylene oxide, which catalyst comprises silver on a porous inorganic refractory carrier containing at least 0.003 gram equivalent, per kg of complete catalyst, of cesium and/or rubidium chemically absorbed on the surface of the carrier and a catalyst wherein the amount of the chemically absorbed cesium and/or rubidium falls in the range of 400 to 3,000 ppm based on the complete catalyst per unit surface area, m.sup.2 /g, of the carrier.
In the specification of No. GB-2117263, there is disclosed a catalyst which comprises a granular carrier made of alumina, silica, silica-alumina, or a combination thereof, possessing a surface area approximately in the range of 0.05 to 1.5 m.sup.2 /g, and having a characteristic ability to absorb selectively an alkali metal from a solution of the alkali metal, 5 to 20% by weight, based on complete catalyst, of a silver dispersion deposited on the granular carrier from a solution of an organic silver salt and activated in the presence of molecular oxygen at a maximum temperature not exceeding 500.degree. C. for a time long enough to produce an active fresh catalyst and consequently allowed to exist in the form of particles of an average particle diameter approximately in the range of 0.2 to 1.0 micron, and at least one alkali metal selected from among cesium, potassium, and rubidium deposited in an amount approximately in the range of 10 to 1,000 ppm by weight based on the complete catalyst on the dispersed active silver particles from a solution composed of water and an alkanol of 1 to 3 carbon atoms.
Numerous reports on silver catalysts have been published as described above. Most of them, however, are directed to improving the performance of silver catalyst by addition to the catalyst of an alkali metal from a specific range. All these catalysts, however, have still many problems in terms of performance and service life as a catalyst.
The effects manifested upon silver catalysts for the production of ethylene oxide by the addition thereto of reaction promoters represented by alkali metals have found recognition widely. They have been disclosed in numerous patented inventions. Most of these inventions, however, are nothing more than empirically unveiled effects. Virtually none of them has gone the length of elucidating an actual chemical action which is responsible for the effect involved. It is well known by persons skilled in the art that, owing to such true state of affairs as described above, in no few patented inventions, the inventors have disclosed contradicting technical concepts. Even in general technical literature, there are found reports such as the report written by Margolis and titled "Catalytic Oxidation of Hydrocarbons" which purport in effect that the addition of alkali metals results in degradation of the selectivity for ethylene oxide, suggesting that test results heavily hinge on methods of test adopted by individual researchers. This situation may well be regarded as a confusion arising solely from incomprehension of the true nature of chemical actions of reaction promoters. We, as the result of a diligent study, have succeeded in elucidating the chemical actions of reaction promoters and, based on the knowledge consequently acquired, perfected a literally ideal catalyst.
Various inventions have been proposed concerning kinds of reaction promoters to be used, amounts of such reaction promoters to be added for effective use, and methods of addition of such reaction promoters. The conditions in which such reaction promoters are distributed in catalysts and the actions of the reaction promoters manifested in the catalysts, however, have not been elucidated. Exceptionally, in the specifications of No. EP-85237 and No. GB-2117263, there are found statements to the effect that chemical absorption or adsorption of alkali metals on carriers is effective. These statements are interpreted as implying that the adsorption poisoning of an alkali metal done to acid sites on a carrier brings about an effect of suppressing the reaction of isomerization of ethylene oxide into acetaldehyde which is a secondary reaction in the reaction for the formation of ethylene oxide. The inventors' study has also yielded results which support these statements. It should be especially noted here, however, that in the conclusion drawn from the inventors' study, the effect of the addition of an alkali metal to the silver catalyst is manifested predominantly on the performance of silver and only secondarily on the adsorption poisoning done to the acid sites on the surface of the carrier. If the adsorption poisoning to the acid sites on the surface of the carrier is ideally realized, it does not necessarily follow that this achievement will consequently bring about a marked improvement of the performance of the catalyst (selectivity for ethylene oxide). We have confirmed that the catalyst's acquisition of an ideal performance is not realized unless the compound such as an alkali metal is deposited in an optimum amount as dispersed on the monomolecular level (with the ions of alkali metal distributed one by one independently) on the surface of silver.
In the specification of No. GB-2117263, there is found statement purporting in effect that the act of intentionally using a carrier possessing numerous acid sites for the purpose of increasing the amount of a metal to be chemically adsorbed on the carrier is effective. We are of an opinion that use of a carrier having numerous acid sites is not beneficial.
Our conclusion has drawn regarding the chemical action of a metal additive manifested on the surface of silver is that the steric hindrance effect on the surface of silver is greater than the electronic effect advocated by Margolis et al.. Margolis et al. predict that the addition of an electron donating alkali metal tends to lower the selectivity for ethylene oxide. This theory evidently contradicts the effectiveness of an alkali metal which has found recognition widely. The primary ground on which the inventors adhere to the steric hindrance effect is the fact that the adsorption such as of an alkali metal on the monomolecular level on silver contributes greatly to the improvement of selectivity. The second ground which supports their conclusion is the fact that, in addition to such heavy alkali metals as cesium, rubidium, and potassium which have already been generally accepted as effective promoters, thallium has been demonstrated by a research group including the inventors to be an equally effective promoter and all these metals possess the large cation radii in common. The four metal ions of cesium, rubidium, potassium, and thallium (monovalent) possess the four largest cation radii in all the elements excepting instable radioactive elements. In such factors as electronegativity, ionization potential, and work function which have bearing on the electron effect, these metal ions having nothing to share in common. The steric hindrance effect on the surface of silver is considered to be manifested conspicuously in the suppression of dissociative adsorption of molecular oxygen and in the suppression of readsorption of produced ethylene oxide and both the forms of suppression are believed to contribute directly to the enhancement of selectivity. Methods which define ranges of amounts of metals to be added per kg of a catalyst and ranges of amounts of metals to be added per unit surface area, m.sup.2, of a carrier which are frequently found in the patented inventions published to date are extremely superficial and far from essential truths. The truth is that the performance of a catalyst is largely swayed by the condition in which a given metal additive is present in the catalyst. This is the very cause for the confusion which has brought about a wide variance among the test results obtained by different researchers. The catalysts prepared in accordance with such conventional techniques as described above are not perfectly satisfactory in performance, particularly in terms of selectivity.
An object of this invention, therefore, is to provide a novel silver catalyst for the production of ethylene oxide and a method for the manufacture of the silver catalyst.
Another object of this invention is to provide a catalyst which is enabled to acquire heretofore unattainable high selectivity and retain this quality for a long time by causing a reaction promoter of a fixed amount relative to the surface area of the silver in the catalyst to be dispersed and deposited fast on the monomolecular level on the surface of the silver and a method for the manufacture of the catalyst.